Helicopter-borne GPR provides insights on the 2024 La Bérarde flood

On 21 June 2024, the mountain settlement of La Bérarde (French Alps) was severely damaged by a flood and debris flow. Preliminary investigations conducted by the local French authorities indicate that flooding was caused by a compound event combining (i) exceptional precipitation amounts, (ii) high snowmelt rates, and (iii) a supraglacial lake outburst flood from the nearby Glacier de la Bonne Pierre. Water balance consideration, however, indicated that additional water might have come from a subsurface reservoir, possibly located within Glacier de la Bonne Pierre. To better asses this possibility, we surveyed the glacier with a dedicated Ground Penetrating Radar (GPR) campaign in November 2024. The survey used the Airborne Ice Radar of ETH Zürich (AIRETH), a dual-polarization, helicopter-borne GPR system that has been successfully applied in previous glaciological studies.

The resulting 20 km of high-quality GPR data allowed detailed imaging of the glacier’s internal structure and bedrock. We focus on the possibility for large water accumulation within the glacier, or at the ice-bedrock interface, that could have been the origin reservoir contributing to the total flood volume. Along the glacier tongue, we detected a widely distributed, high scattering zone, indicative of temperate ice containing small water inclusions. Additional evidence of a subglacial drainage network was observed, supported by both peculiar GPR features and the presence of a river outcrop at the glacier tongue, as well as possible cavities within the ice. We applied the VAW-ETHZ package WhereTheWaterFlows.jl, which determine subglacial water flow paths, to further investigate the subglacial drainage of the glacier and validate the GPR interpretation. The outcomes of this study demonstrate the value of combining advanced geophysical techniques with modelling approaches to deepen the understanding of glacier-related hazards.